[R] different values under the x axis labels

[Robert Baer]

I have a .xls source file from 1 to 3600 seconds, each second has a noise 
level value. (attached in case you are interested)
Imported as "Dataset".
I would like to make a graph: Here is what I have been able to use:

par(mar = c(5, 4, 4, 2) + 0.1)
plot(Dataset, type="l", xlab="čas [s]", ylab= expression(paste("Hladina 
akustického tlaku","     ", bold(L[pA])," [dB]")), col="blue", 
xlim=c(0,3600), ylim=c(35,60), xaxs = "i", yaxs = "i",cex.main=1.5, 
cex.lab=1.2,xaxt="n")
abline(h=43,col="red",lwd=2)
title(main = "Místo příjmu 3, čas 9:00 - 10:00",font.main="2", line=3)
axis(3, xaxp=c(0, 3600, 12), mgp=c(3,0.6,0))
axis(1, xaxp=c(0, 3600, 12), mgp=c(3,0.6,0))
smartlegend(x="left", y="top",c(expression(paste(bold(L[eqA.1h]),bold(" 
[dB]"))), expression(paste(bold(L[pA]),bold(" [dB]")))),col=c("red","blue"), 
lty= 1:1,lwd=2, y.intersp= 1.1, bg="grey96",cex=1.1)

I succeeded in changing scale on both "X" after 300 seconds and now I would 
like to swap the value 300, 600, 900 (seconds)of the top axis to 5, 10, 15 
(minutes) and so on OR
the best of all would be to have these numbers (minutes) under the original 
bottom x axis values in different color.

Does this get you started?

Dataset=data.frame(time=1:3600,noise=15*abs(rnorm(3600)))
plot(Dataset, type="l", xlab="čas [s]", ylab= expression(paste("Hladina 
akustického tlaku","     ", bold(L[pA])," [dB]")), col="blue", 
xlim=c(0,3600), ylim=c(35,60), xaxs = "i", yaxs = "i",cex.main=1.5, 
cex.lab=1.2,xaxt="n")
abline(h=43,col="red",lwd=2)
title(main = "Místo příjmu 3, čas 9:00 - 10:00",font.main="2", line=3)
# axis(3, xaxp=c(0, 3600, 12), mgp=c(3,0.6,0))
axis(3, at=seq(0,3600,300), labels=seq(0,60,5))
axis(1, xaxp=c(0, 3600, 12), mgp=c(3,0.6,0))


Rob